Rapid and reliable measurement of water quality is of major importance, particularly with respect to drinking water. Ultraviolet transmittance (UVT) is a water quality parameter that provides a measure of the amount of ultraviolet (UV) light able to transmit through a water sample. The ultraviolet absorbance (UVA) is a different representation of the measurement of UVT. UVA is mathematically related to UVT by the inverse log.
The ultraviolet transmittance (UVT) of a water sample under test (test sample) is a measure of the transmittance of water to UV light. Fundamentally, this requires a UV light source to shine UV light through a test sample and into a UV detector. However, to properly calculate the UVT of a test sample, the amount of UV light that is able to pass through the test sample must be compared to a reference of some kind. The reference is generally a sample of pure water which is said to have a UVT of 100% (blank sample), but any form of reference having known UVT could be used. By comparing the amount of UV light able to pass through the test sample to the amount of UV light able to pass through the blank sample, a useful value of UVT can be calculated for the test sample as a fraction or percentage. From the value of UVT, the UVA may then be calculated. There are many different UVT/UVA measuring devices available today that are able to measure the UVT/UVA of a test sample as compared to a blank sample using many different technologies and configurations.
There are two main types of UVT/UVA measuring devices. The first type is considered to be portable, although it may be permanently mounted as a benchtop instrument. It is designed to be operated by a user taking the UVT/UVA of grab samples and is typically used in water or chemical analysis labs or as a water or chemical analysis tool in the field. The second type is considered to be online such that it is directly connected to an incoming water source and continuously calculates the UVT/UVA of the incoming water. It is typically found in municipal water and wastewater treatment plants and industrial process water applications.
There are two main challenges when designing UVT/UVA instrumentation. The first challenge is due to the nature of UV light sources. The most common UV light source is the mercury lamp, which has a tendency to drift and fluctuate causing significant errors in the UVT/UVA measurements. Such fluctuation and drift is very common in UV lamps and is due primarily to changes in temperature and imperfections in the ballast and lamp, as well as the age of the lamp. Another major difficulty when designing UVT/UVA instrumentation is due to fouling of the optical path by various types of matter in the water. Dirt, oil and minerals can be deposited by the test water on optical windows and even on the UV detector and lamp. This deposition can significantly impair the UV light's ability to transmit to the sensor thereby causing significant errors.
For the above reasons it is necessary to recalibrate the UVT/UVA measuring device as frequently as possible in an attempt to reduce these errors.
The use of a blank sample for calibration, while potentially effective, causes various problems for both portable and online UVT/UVA measuring devices. For portable devices, the use of a blank sample typically requires filling a sample vial with the blank sample and performing a calibration procedure with the device followed by filling the sample vial with the test sample and performing a test procedure with the device. While the use of a blank sample as a reference for UVT/UVA calculations is acceptable in the lab it is not desirable in the field. Carrying blank samples in the field can be cumbersome and can cause problems in harsh climates where temperature and freezing can affect the UVT/UVA of the blank sample.
For online UVT/UVA measuring devices, the use of a blank sample is especially problematic since these devices generally require a constant flow of test water through the flow cell through which the UVT/UVA is measured. Typically, in order to calibrate a conventional online device the flow cell must be disconnected from the incoming test water, the flow cell must then be emptied of test water and replaced with water from a blank sample, then the calibration procedure must be performed, then the blank sample water must be removed from the flow cell and the incoming test water must then be reconnected to the flow cell. Clearly this is a time consuming process and prone to human error if performed by an operator. Some devices attempt to automate this process, however this requires additional fluid handling apparatus which makes the device both more expensive and bulkier.
Even if online UVT/UVA measuring devices do use an automatic blank sample calibration apparatus, the frequency that it is practically possible to calibrate is often only a few times per day at most, which is not nearly enough to prevent errors due to lamp fluctuations
Newer designs have recently been introduced that allow the calculation of the UVT/UVA of test water without the need for blank samples, which is a significant improvement. The newer designs use a method of calculating the UVT/UVA by measuring the transmittance of light through different path lengths of the test water. By measuring the transmittance through at least two different path lengths of test water it is possible to compute the UVT/UVA of the test water while calibrating at the same time. The calculations required to determine the UVT/UVA using different path lengths depend on the number of path lengths used and what the path lengths are.
There are many different ways to design a device that measures UVT/UVA using multiple path lengths. The primary challenges all relate to difficulty implementing the different path lengths.
Several different approaches have been taken in the past. Some devices use a single UV detector and lamp, and by changing the relative position of the UV detector and lamp, two or more different path lengths may be defined. U.S. Pat. No. 6,818,900 describes such a device. However, there are several problems with this design. It is extremely important that the path lengths used are always consistent. This requires the positioning mechanism that defines the different path lengths to be highly accurate which adds to the cost of the device. Also, since the path lengths must be precisely known, this design also requires some form of path length factory calibration procedure which again adds to the cost of producing such a device. Also, this design requires that the test water chamber must contain moving parts. This requires water tight seals to be used which adds to the expense and also the maintenance of the device.
Other multiple path length designs require the use of multiple UV detectors. By simply fixing each UV detector a certain known distance from the lamp each UV detector is able to define a different path length. This allows the device to be designed with no moving parts. U.S. Pat. No. 6,791,092 describes such a device. However, this method introduces new errors due to the use of multiple sensors. Manufacturing is very costly since the relative distance between the lamp and each sensor must be very precise. Differences in the optics of each UV detector location can produce non-linear differences between the measurements made using each sensor. Differences in the electronic signal path of each UV detector can also significantly affect the measurements of each detector. Also, if each detector is looking at a different part of the lamp and/or looking at the lamp from a different angle, errors can occur since the UV lamp output varies not only over time, but also over the surface of the lamp. Therefore, using multiple UV detectors can reduce the effectiveness of the fundamental multiple path length concept.
Therefore, there is a need for a UVT/UVA measuring device which utilizes a multiple path length design while avoiding the aforementioned limitations.